The present invention relates to a current sensing circuit for an Asymmetric Digital Subscriber Line (ADSL) interface circuit and an ADSL interface circuit.
An ADSL system transmits data at a high rate over a conventional telephone line. To do this, it employs a frequency band from 26 kHz to 1.108 MHz. This band is divided into sub-bands of 4.35 kHz width, each of which is modulated in a Quadrature Amplitude Modulated (QAM) pattern to carry up to 15 bits of data. The data is refreshed at 250 micro-second intervals.
The intensive signal processing required for such data communication is performed in dedicated DSP integrated circuits. These circuits, which require several million transistors, are constructed with very high density integrated circuit processes, which processes can only support low voltages e.g. 3.3 V. An ADSL interface circuit provides the interface between the low voltage DSP circuits and the telephone line.
In order to provide adequate signals at the receiving end of the telephone lines, which may be several kilometers long, the ADSL signal at the transmit end must be relatively large. Typical specifications require an average power level of 20 dBm on a 100 xcexa9 line, which means the ADSL signal must be 3.16 Vrms and 31.6 mArms. The true requirement is, however, more stringent because the ADSL signal may have peak values which are very much higher than the rms level. In fact, the peak is typically limited at a value which is significantly below the theoretical maximum, but which, the signal statistics show, will be exceeded with an acceptably low probability. In a known case, this level is 17 dB above the rms voltage, i.e. 22.4 Vpk, which will, of course generate 224 mApk current.
The high density DSP circuit will normally comprise distinct paths for transmit and receive signals. The two-wire telephone line, however, must carry these signals simultaneously. In order to separate the received signal from the transmitted signal, the ADSL interface circuit therefore includes a hybrid balance circuit which should be arranged so that in operation there is matching between a reference impedance determined by the interface circuit and that of the telephone line.
The ADSL interface circuit must also present a definable impedance to the telephone line, both to preserve a correct frequency response in the transmit and receive paths and to ensure, in some conditions, that the impedance presented at the other end of the telephone line is well defined.
The high frequency channels of the ADSL signal may be considerably attenuated on long telephone lines. They are consequently very sensitive to degradation by spurious signals which could be generated from the lower frequency channels by intermodulation distortion.
Taking all the above into account, there is therefore a need for an ADSL interface circuit which can accept a data signal from a low voltage circuit, amplify and transmit the data signal at a high voltage level via a telephone line and which can simultaneously receive a high voltage level data signal from the telephone line, adjust the received signal to an appropriate level and transmit it to the low voltage circuit. In addition, the ADSL interface circuit should have very low distortion and should provide impedance matching and a definable impedance to the telephone line.
The requirements of the ADSL interface circuit are similar to those of the Subscriber Line Interface Circuits (SLIC""s). Examples of SLIC arrangements can be found in European patent application no. EP-A-467367 and US Pat. No. 4,622,442. SLICs are used for voice communication systems and therefore do not address the specific requirements associated with communicating data at high data rates. For example, SLICs do not have stringent requirements for low distortion at high frequencies which means that they are able to reconstitute line current signals which are required, together with the line voltage signals, to generate the aforementioned defined impedances from images of the currents of a plurality of class AB output stages. This technique for sensing the line current signals would be inadequate for an ADSL interface circuit because the matching required could not be maintained over the frequency band, due to the large operating current range of the class AB output stages and their differing stray capacitances.
Implementing such a circuit with discrete passive components, for example using a resistor coupled across the line to achieve impedance matching, is also not a satisfactory solution because of the amount of power which would be consumed by such a solution.
Classic hybrid balance circuits, for example those used in telephone handsets, are DC coupled to the telephone line and thus cannot be used in an ADSL system which, because the system is coupled to the mains supply, requires a transformer to couple the interface circuit to the line.
In accordance with the present invention there is provided a current sensing circuit as recited in claim 1 of the accompanying claims.